For digital systems, accurate timing is crucial to data transmission. Clocking signals therefore are crucial to digital systems, because clocking signals set the timing for the components in the systems. A computer motherboard, for example, might have a single master clock signal that is transmitted to and synchronized with integrated circuit boards, chipsets, peripherals, or other components connected to the motherboard. All system components may be synchronized using this master clock signal.
Various techniques exist for generating and distributing clock signals within a digital system. For example, a primary clock signal might be generated by a ring oscillator or a separate clock chip (e.g., a crystal oscillator). The clock signal may then be routed from the generator to each of the devices connected to the clock. These techniques use electrical clock signals, i.e., clock signals traveling along metallic or semiconductor conduits. Unfortunately, clock signals in the electrical domain present numerous design limitations.
Ideally, clock signals would have a well defined duration, consistent shape, and zero propagation path dependence. In reality, clock signals have variable rise and fall times, noticeable jitter, and noticeable path-dependent skew, a particular problem that arises from timing differences and waveform variation between clock signals. There is also a sizeable power drain associated with electrical clock signals.
Typically, clock signals are distributed throughout a system via a distribution network. In theory, the network would make duplicate copies of a clock signal and provide identical paths for each duplicate copy, so that any device connected to the network would receive a synchronized clock signal. In reality, however, skew and jitter problems abound, primarily due to electrical load differences among the various paths and parasitic effects within the network.
Recently, some have proposed moving away from a purely electrical domain digital clocking system to an optical domain clocking system. Using optical signals, i.e., light pulses, presents some obvious theoretical advantages. Optical signals are not susceptible to load variations or parasitic effects, because they travel through waveguides and not conducting metallic wires. Also, optical signals may propagate at much faster clock rates. Electrical clock signals have a theoretical limit of about 25 GHz for signal transmission of about 5 mm, while optical clock signals may extend into the THz range and travel much longer distances, thus allowing for a digital system with orders of magnitude faster performance capabilities.
In the optical clock networks proposed for clock signal distribution, a network distributor generates or receives a clock signal, and that signal is then split into multiple signals by a Y-branch splitter, multimode interferometer or similar device. Each copy of the clock signal is then provided to one output waveguide, where all the output waveguides are of equal length to keep the copies of the clock signal in phase.
While optical networks do not have the impedance load variation and parasitic problems of electrical domain networks, they have their share of shortcomings. One of the main problems affecting optical networks is modal confinement. For an optical signal to propagate and not lose its waveform or intensity, the signal's mode must be confined to the propagating waveguide. Further still, its mode profile must stay constant over the propagation length of that waveguide. This means that the waveguides must have a higher index of refraction differential with respect to their surrounding cladding layers. This also means that only waveguides of certain bending radii (typically quite large) are used to avoid bending losses. Unfortunately, large bending radii result in large devices and, as such, limit device scalability. The problem is multiplied with network complexity.